Maintenance and Rehabilitation strategies for asphalt pavement
When it comes to managing pavement, the decision to opt for maintenance or rehabilitation is generally determined based on the current condition of the surface and structure. For instance, if the surface condition is deemed to be of a satisfactory level, maintenance may be the preferred option. However, if the condition falls below a certain threshold, both maintenance and rehabilitation could be required. In such cases, the selected course of action will depend on the pavement's structural state and may range from significant maintenance work, like a thin overlay, to full-scale rehabilitation, such as reconstruction. The chart provided below identifies the different treatment options for both maintenance and rehabilitation.
The sections below go into further detail about the treatment techniques for flexible pavement.
Maintenance
Adequate maintenance can increase the lifespan of pavements and reduce the cost of future repairs. This need for proper maintenance is globally recognised, and efforts are underway to develop improved techniques for selecting appropriate methods and new approaches.
Primary Corrective Maintenance activities
Crack sealing is a process of filling up working cracks in pavement with specialized materials to prevent the intrusion of incompressible materials and water. To ensure effectiveness, it is important to select an organisation-approved sealant product that matches the temperature and traffic, and to apply it properly on a clean and dry surface using equipment that’s in good working condition. Crack sealing should be done in dry weather conditions with a minimum temperature of 4 degrees Celsius and rising.
On the other hand, crack filling involves placing materials into nonworking cracks in pavement, which have less than 2mm of horizontal movement. This process helps to reinforce adjacent pavement and substantially reduce water infiltration.
To repair potholes, it is important to use high-quality hot or cold mix with enough binder for workability, compaction, and durability. The potholes should be cleaned and dried, and a tack coat applied before the patching material is placed. The patching material can be locally available or produced according to specifications. The compatibility between the aggregate and asphalt binder must be ensured. The conventional method involves placing and compacting the materials with truck tyres to create a 3-6mm crown on the pothole. Before placing the materials, the pothole should be cleared of water and debris, and its sides cut to vertical surfaces. Tack coat should be applied on the sides, and a small roller or plates used for compaction. There are many spray injection techniques available for pothole patching.
2. Primary Preventive Maintenance Activities
Slurry seal: A slurry seal is a mixture of fine aggregates, mineral filler, and diluted asphalt emulsion with additives such as hydrated lime. The mix is made on-site by a slurry machine and applied with a squeegee. Slurry seal effectively seals minor cracks, reduces the potential of ravelling, and improves pavement friction properties. The mix is specific to pavement type with different types of traffic levels, and requires 2-6 hours of curing time. It is not suitable for heavily cracked pavements and requires caution in areas with superelevation. Compaction may be necessary using a light rubber-tyred roller.
Chip seal: A chip seal involves applying an asphalt binder or rapid-setting emulsion followed by an aggregate layer, which can be a single layer or multiple layers with finer gradations in each successive layer. Compaction is necessary, and the minimum pavement temperature should be 15 degrees Celsius. It is suitable only for pavements with light traffic. Also, windshield damage can occur if the aggregates are not embedded properly. A rubberised chip seal with ground tyre rubber offers better resistance to climate and traffic-induced stress and minor fatigue cracks. It is known as a stress-absorbing membrane (SAM) or stress-absorbing membrane interlayer (SAMI) when used under an HMA overlay.
Sandwich Seal: This method involves the application of a chip seal with two layers of aggregate and an asphalt layer in between, resulting in improved frictional properties.
Cape Seal: A cape seal involves applying a chip seal followed by a slurry seal, preventing loose stones in chip seals, and is suitable for pavements with high traffic volumes. However, it requires a curing period of 2-6 hours.
Fog Seal: A fog seal is a method of applying a diluted asphalt emulsion (slow or medium setting) without any aggregate to prevent ravelling and oxidation of the surface layer. This method requires a temperature above 16 degrees Celsius for application and needs time to cure before opening to traffic. It may temporarily reduce friction until the surface wears away from traffic. A sand seal, where sand is applied over a rapid or medium-setting emulsion layer, can improve friction.
Microsurfacing: Microsurfacing is a method of pavement treatment that involves applying a mixture of high-quality aggregates and polymer-modified emulsion binder with additives. This process is used to seal the pavement, prevent ravelling, resist oxidation, improve friction, and fill in minor ruts. The mixture is produced in a travelling pug mill and does not require any compaction in areas with traffic. Microsurfacing uses fast-setting emulsions and needs mix design for determining asphalt content, as well as advanced equipment for application.
Thin HMA overlay: A thin HMA overlay, also known as a porous friction course (PFC), is typically 1.3 to 2.5 cm thick and consists of gap-graded aggregates and polymer-modified asphalt binders. These overlays are designed to promote rapid drainage of water and improve friction between the pavement and tyres. They can contain ground tyre rubber and high asphalt contents for added durability. Compaction is not required, but cracks must be sealed and the underlying layers must have relatively low voids.
Ultrathin friction course: The ultrathin friction course is a process that applies a polymer-emulsion asphalt HMA layer in one pass using specialised equipment. This process is used for surface sealing, minor crack filling, and improving friction and decreasing tyre noise.
Recycling
To recycle asphalt, there are several methods, including hot mix recycling, hot in-place recycling (HIR), cold in-place recycling (CIR), and full-depth reclamation (FDR). Hot mix recycling is widely used for producing hot mix asphalt, while hot in-place and CIR are used for preventive maintenance. Full-depth reclamation is used mainly for rehabilitation work.
Hot in-place recycling
Hot in-place recycling is an on-site method used to rehabilitate deteriorated asphalt pavements. This process consists of softening the asphalt surface, scarifying and/or mechanically removing the surface material, mixing it with a recycling agent or new mix, and laying down the recycled mix on the pavement surface. By using this process, the use of new materials is minimised.
Hot in-place recycling is a process that rehabilitates deteriorated asphalt pavements, correcting surface distresses like rutting, ravelling, cracks, and holes. The process can be done in a single-pass or multistep operation. Hot in-place recycling has advantages such as preserving elevations and overhead clearances, being economical, and requiring less traffic control. It can also modify aggregate gradation and asphalt content and improve surface frictional resistance. The typical depth of this process is 25mm, and it is commonly used for preventive maintenance operations.
2. Cold Recycling
Cold recycling can be categorised into two types: cold in-place recycling (CIR) and central plant (cold mix) recycling. CIR can be further divided into two types: CIR for the asphalt surface or surface plus binder layer, and full-depth reclamation, which involves recycling of the asphalt layer and some part of the granular base material. Cold milling is also used to gather materials for hot mix recycling.
Cold Milling
Cold milling is a process involving the automatic but controlled removal of pavement to a predetermined depth with equipment specialised equipment, as well as restoring pavement surfaces to a specific grade and slope, removing all bumps, ruts and other defects.
Modern cold milling involves the use of specialised equipment with tungsten carbide teeth drums that can remove pavement to a desired depth and restore the surface to a specified grade and slope. The equipment has variable cutting width and can be used in difficult milling situations. Cold milling equipment can also be integrated with a recycling system that sprays recycling or stabilising materials, mixes, and compacts the material.
Cold in-place recycling
Cold in-place recycling (CIR) is a pavement rehabilitation technique that involves reusing existing pavement materials in place without the use of heat. This is achieved by milling, planning, or crushing the existing pavement to obtain reclaimed asphalt pavement (RAP) material, which is then mixed with virgin aggregate or recycling agent or both, and then laid and compacted. The use of CIR can restore old pavements to their desired profile, eliminate wheel ruts, restore the crown and cross slope, and eliminate potholes and rough areas. Some of the advantages of CIR include its potential for cost savings, high production rate, minimum traffic disruption, retention of the original profile, and reduction of environmental issues.
The steps in CIR include preparation, milling of existing pavement, addition of recycling agent and virgin materials, laydown, compaction, and placement of the surface course. The average speed of the process is about 4.6 to 18.3 meters/min. Overlap during multiple passes can be managed by turning off some sprayer nozzles. Compaction is done using vibratory drum.
Adding new aggregates might not be necessary in some projects. Single machine or single-pass equipment can be used for CIR, which is able to break, pulverise and add in recycling agents in a single pass.
Full-depth reclamation
Full-depth reclamation is a recycling technique that involves treating the entire section of asphalt pavement, and a pre-established quantity of underlying materials to produce a stabilised base course. Additives such as asphalt emulsions, calcium chloride, Portland cement, fly ash and lime are added to improve the base. The five key steps in this process are pulverisation, introduction of additive, shaping of mixed material, compaction, and application of a surface or wearing course. This technique is recommended for pavements with deep rutting, load-associated and non-load-associated cracks, and maintenance patches. It is usually performed to a depth of 100-300mm.
Multi Recycling Stabilisation
In cases where a thick layer of HMA over a poor-quality base material is causing repeated pavement distress, a combination of recycling work or multi recycling/stabilisation may be necessary. A good example is the rehabilitation work done on a section of the I91 in Virginia, USA. The top 25 cm of the section was milled off, leaving only 2.5 cm of HMA on the base course. The milled material was then recycled in a central cold recycling plant with foamed asphalt, while the remaining HMA layer and base underwent Full Depth Reclamation with lime up to a depth of 30 cm. The foamed asphalt recycled material was placed over the FDR reclaimed base and overlaid with two 5 cm thick lifts of HMA.
3. Hot Mix Recycling
Hot mix recycling involves combining RAP with new aggregate and an asphalt cement or recycling agent to create HMA. RAP can be acquired through pavement milling using a rotary drum cold-milling machine, or from conducting rippling/crushing operations.
The RAP can be used in various ways, including being an addition to regular HMA, being used as an aggregate in cold-mix asphalt, as a pulverised granular base course, or as a fill or embankment material.
While there are variations in the process, the centre entry method is most commonly used approach for hot mix recycling in a drum mix plant. The RAP material is heated through conduction by hot virgin aggregates. To protect the RAP from direct contact with the burner flame, a dense veil of aggregate is added prior to the point where the RAP is introduced. The presence of the virgin aggregate veil is crucial in preventing RAP overheating and the resulting "blue smoke," which can make it impossible to use the intended amount of RAP material. To address this issue, special flight design, steel ring dams, or circular steel flame shields can be employed to ensure proper mixing of the RAP with the virgin aggregates before subjecting it to high gas treatment, thereby eliminating the "blue-smoke" problem.
Conclusion
Flexible pavement maintenance and rehabilitation strategies are critical for ensuring the longevity and functionality of roadways. These strategies involve a range of approaches, including preventive maintenance, corrective maintenance, and rehabilitation techniques such as hot mix recycling, full-depth reclamation, and overlaying. Properly executed maintenance and rehabilitation programs can help reduce pavement distress, improve ride quality, and extend the lifespan of roadways, resulting in cost savings and improved safety for motorists.
Consult with our pavement experts to make an informed decision on the most suitable treatment option. Click here for more information on our pavement consultancy services.